The continuous operation of fixed bed reactors in the hydrotreatment of hydrocarbon fractions is limited by the lifetime of the catalyst within the fixed bed and the pressure drop across the fixed bed. Both these factors are affected by metallic contaminants within the hydrocarbon fraction to be treated. Metallic contaminants such as iron, vanadium, arsenic and nickel compounds are well known as catalyst poisons, drastically reducing the useful lifetime of hydrotreating catalysts. Suspended metallic material may also be deposited within the catalyst bed, blocking catalyst pores and voids between catalyst particles and restricting the flow of oil and gas, leading to pressure drop.
Processes for removing solid as well as dissolved metallic contaminants from hydrocarbon fractions have been described in the art. For example, in EP-A-0,399,592 a hydrotreating process is disclosed for the removal of solid and dissolved metallic contaminants using as the catalyst porous inorganic oxide particles having a surface area of at most 1 m2/g and a pore volume of at least 0.1 ml/g in pores having a diameter of at least 10 microns. The inorganic oxide may additionally contain oxides or sulphides of metals of group VA, VIA and/or VIII. JP-A-05/184941 describes a hydrotreating process for removing solids and dissolved contaminants, wherein a catalyst is used having a surface area above 5 m2/g, a pore volume of 0.1 cm3/g in micropores having a diameter of at least 0.004 micron (40 Å) and a void ratio of 30 to 70%. The catalyst may also additionally contain oxides or sulphides of metals of group V, VI and/or VIII. In U.S. Pat. No. 3,947,347 a process for removing metallic contaminants from a hydrocarbon feedstock is disclosed, in which process a catalyst is used which is free of any hydrogenating component and which has a surface area of from 1 to 20 m2/g and pores having an average diameter of from 0.1 to 1 micron (1,000 to 10,000 Å). EP-A-0,714,699 discloses a catalyst comprising both macroporosity and microporosity for the removal of both solid and dissolved metallic contaminants from a hydrocarbon oil.
Higher surface area catalysts and support materials are disclosed inter alia in NL-A-8,403,107 and U.S. Pat. No. 3,853,789. NL-A-8,403,107 discloses a high surface area (in excess of 100 m2/g) catalyst, which has feeder macropores connecting to catalytically active micropores for metals and sulphur removal from asphalt-containing hydrocarbons. U.S. Pat. No. 3,853,789 discloses high surface area (200 to 400 m2/g) alumina material with a substantial macroporosity.
Also described in the art are methods of producing porous hydrogenation catalysts having a bimodal pore distribution, wherein the macropores or channels are provided by the incorporation of removable carbon-containing substances into the alumina support and their subsequent removal by heating or another method. For example, GB-A-867,247 describes a method for the production of a catalyst, comprising pores with diameters in the range of from 4000 to 15000 nm, by mixing 1 to 2 wt % of cellulose fibres with alumina/silica before forming particles from the mixture and calcining the particles to remove the fibres and form a catalyst support. GB-A-943,553 is directed to a process for hydrogenative refining by contacting hydrocarbon fractions with a catalyst produced by the incorporation of metals from group VIA and VIII of the periodic table onto such a support. DE-A-1,803,773 is directed to improving the lifetime of a hydrotreating catalyst by producing it in such a manner that the catalyst particles comprise a large number of randomly oriented channels, with diameters in the range of from 0.1 to 100 μm, essentially uniformly distributed in the particle and providing in the range of from 0.05 to 0.10 cm3/g of the total pore volume of the catalyst particles. These channels are formed by incorporating fibres into a mixture of catalyst components before forming particles and then destroying the fibres. The production of a hydrotreating catalyst with a bi-modal pore distribution comprising 10% of the pore volume in macropores in the range of from 200 to 1500 nm is described in U.S. Pat. No. 4,448,896. Such a pore distribution is achieved by the incorporation of carbon black into the alumina of the catalyst before forming shaped particles and then calcining them in an oxygen-containing stream to burn off the carbon black.
Regardless of the advances described in the prior art, there still remains the need for improved catalysts for the demetallisation of hydrocarbon fractions. It is the aim of this invention to provide a catalyst with an increased uptake capacity for metallic contaminants thus allowing longer operation times and lower pressure drops when used in a process for hydrotreating hydrocarbon fractions comprising such contaminants.